The goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One crop species which has been subject to such breeding programs and is of particular value is the cucumber. Cucumber (Cucumis sativus L.) is naturally a diploid (2n=14) outcrossing species originating in Asia or Africa, although haploid, doubled-haploid (U.S. Pat. No. 5,492,827), and triploid (Sarreb et al., 2002) types have been developed. Humans have cultivated cucumbers for several thousand years. The cucumber has large leaves that form a canopy over the fruit. The vine is grown on the ground or on trellises. The two main types of cucumber fruit grown commercially today in the United States are fresh market (slicing) type and the processing (pickling) type. Varieties and production methods are typically adapted to the end use. Slicing cucumbers are often longer, larger and have darker and thicker skin, whereas pickling/processing cucumbers have a shorter fruit, thinner skin with interior flesh that makes them more amenable to pickling. Seedless varieties are generally preferable for both fresh market and for pickling as developing and large seeds are not palatable.
Until the 1960s cucumbers were normally monoecious, i.e., having separate male and female flowers on the same plant. Perfect flowers are uncommon in cucumbers. Staminate flowers are typically single and/or in clusters. Pistillate flowers may be solitary or in clusters and are borne on stout peduncles. Gynoecious cucumber plants have now been identified in which flowers are exclusively pistillate. These plants are generally higher yielding, due at least in part to the presence of higher numbers of female flowers. However, growth of gynoecious hybrid plants in the field has historically required the addition of plants of a monoecious line or variety (˜10-15%) to ensure availability of pollen and setting of fruit with seed. Honey bees are the most commonly used insects to pollinate cucumbers in the open field.
Cucumber plants that set fruit parthenocarpically (without pollination and fertilization) have more recently been available. These plants produce seedless fruit unless pollinated. Growth of parthenocarpic varieties is beneficial in that setting of fruit on these cultivars does not produce an inhibiting effect on plant growth, unlike the case of fertilized, seeded fruit. The seedless varieties are usually higher yielding and of higher quality due to the lack of seeds. However, growth of these plants requires isolation from seeded cucumbers to avoid pollination and subsequent seeded fruit.
Most of the cucumbers currently used which are processed to pickles and pickle products in the United States are seeded hybrid varieties. Hybrid varieties offer the advantages of easy combination of dominant and recessive traits, such as disease resistance, from a set of inbred parents, as well as careful control of parentage. The production of F1 hybrid cucumber seeds from a pollen parent bearing only male flowers has been reported (U.S. Pat. No. 4,822,949).
Many different cucumber cultivars have been produced, and cucumber breeding efforts have been underway in many parts of the world (see, e.g. U.S. Pat. No. 6,765,130). Some breeding objectives include varying the color, texture and flavor of the fruit. Minimizing the occurrence of bitterness in cucumbers is one such example. Other objectives include optimizing flesh thickness, solid content (% dry matter), and sugar content. Also, breeding programs have focused on developing plants with earlier fruit maturity, more restricted vine growth, improved disease resistance or tolerance, and improved adaptability to environmental conditions.
Advances in biotechnology have resulted in genetically engineered cucumber plants with improved disease resistance. For example, cucumbers resistant to CMV have been developed by expression of CMV protein coat genes (see e.g., U.S. Pat. No. 6,342,655, U.S. Pat. No. 6,127,601, U.S. Pat. No. 5,623,066, U.S. Pat. No. 5,349,128). Transgenic plants exhibiting, for example, other viral resistance traits or high levels of superoxide dismutase have also been reported (U.S. Pat. No. 6,015,942; U.S. Pat. No. 6,084,152).
While breeding efforts to date have provided a number of useful cucumber lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.